Engine out cold-start hydrocarbon emissions generated before light-off of an exhaust system catalytic converter are responsible for a large percentage of the total exhaust hydrocarbon (HC) emissions. Accordingly, to meet stringent federal government emissions standards, engine exhaust systems have been developed that utilize hydrocarbon retaining devices, such as hydrocarbon traps, to retain cold start emissions for later reaction (HC storing), or for recirculation into the engine intake system (HC purging).
However, the inventors herein have recognized several problems therein. As one example, engines may operate with varying fuel compositions thereby producing exhaust emissions that may have different adsorption and/or desorption characteristics. For example, flex-fuel vehicles that operate on fuel having a varying alcohol composition (e.g., ranging from gasoline (E0) to ethanol (E85)) may have significantly different exhaust compositions. In one example, the smaller hydrocarbon molecule content (that is HCs with two or three carbon atoms) generated in the exhaust for E85 may be significantly higher (80% by volume) than that generated in the exhaust for gasoline (50% by volume). As such, the low adsorption efficiency and low retention temperature for such small HCs, such as ethylene, by commonly used HC trap adsorbents, makes their removal from engine exhaust difficult. The irreversible adsorption of larger HCs (that is HCs with more than three carbon atoms) to commonly used hydrocarbon trap adsorbents with high adsorption efficiency makes purging of such large HCs, such as toluene and iso-octane, difficult, leading to coking and blockage in HC traps.
In one example, some of the above issues may be addressed by a method comprising flowing exhaust gas over a hydrocarbon trap, the trap having a plurality of layers of different porosity and/or of a different adsorptive affinity to selected hydrocarbon chains, and flowing hydrocarbons from a fuel tank over the hydrocarbon trap.
In another example, some of the above issues may be addressed by a method of operating an engine having a hydrocarbon retaining system coupled to an engine exhaust, an engine intake, and a fuel tank, said hydrocarbon retaining system including a first zeolite layer with a first pore size, and a second zeolite layer with a second pore size, said first pore size being smaller than said second pore size. The method may comprise, during an engine cold start, routing exhaust gas to the hydrocarbon retaining system to store hydrocarbons in the hydrocarbon retaining system. The method may also comprise flowing hydrocarbons generated from fuel vapors of the fuel tank to the HC retaining system for storage in the HC retaining system. The method may further comprise, during a purging condition, purging the hydrocarbon retaining system with at least a purging gas, and directing the purging gas to the engine intake.
As described herein, in one example, layered zeolites of differing porosity may be used to trap HCs of differing carbon-chain length. By providing differing porosity zeolites in different layers, and by further including differing chemical and physical adsorptive functions, large hydrocarbons may be physically adsorbed for improved purging while smaller hydrocarbons may be chemically adsorbed for improved storing. In doing so, the temperature range for storing and purging operations may be brought within a desired range. By using a layered approach, storing and purging may occur with gas flow in the same direction. However, reversing flow systems for storing versus purging may also be used. Alternatively, a staged approach may also be used. Finally, in one example, by positioning the smaller porosity layer below the larger porosity layer, an upper layer of larger pore zeolites may provide protection to the lower layer of smaller pore zeolites from large hydrocarbon species during both HC storing and/or purging operations.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.